Conversion of hydrocarbons



Reiuued June 25, 1940 UNITED STATES CONVERSION 01" HYDROCARBONB AristidV. Grolc, Chicago, Ill. amlgrior to Univernal Oil Products Company,Chicago, 111., a corporation of Delaware No Drawing. Original No.2,124,567, dated July 26, 1938, Serial No. 105,717, October 15, 1936,Application for reissue September 15, 1939,

Serial No. 295,118

8 Claims.

This invention relates particularly to the conversion of straight chainhydrocarbons into closed chain or cyclic hydrocarbons.

More specifically, it is concerned with a process involving the use ofspecial catalysts and specific conditions of operation in regard totemperature, pressure and time of reaction whereby aliphatichydrocarbons can be emciently converted into aromatic hydrocarbons.

In the straight pyrolysis of pure hydrocarbons or hydrocarbon mixturessuch as those encountered in fractions from petroleum or other naturallyoccurring or synthetically produced hydrocarbon mixtures the reactionsinvolved which produce aromatics from paraflins and olefins are of anexceedingly complicated character and cannot be very readily controlled.

It is generally recognized that, in the thermal decomposition ofhydrocarbon compounds or hydrocarbon mixtures of relatively narrow rangethat whatever intermediate reactions are involved, there is an overallloss of hydrogen, a tendency to carbon separation and a generally widerboiling range in the total liquid products as compared with the originalcharge. Under mild cracking conditions involving relatively lowtemperatures and pressures and short times of exposure to crackingconditions it is possible to some extent to control cracking reactionsso that they are limited to primary decompositions and there is aminimum loss of hydrogen and a maximum production of low boilingfractions consisting of compounds representing the fragments of theoriginal high molecular weight compounds.

As the conditions of pyrolysis are increased in severity using highertemperatures and higher times of exposure to pyrolytic conditions, thereis a progressive increase in loss of hydrogen and a large amount ofsecondary reactions involving recombination of primary radicals to formpolymers and some cyclization to form naphthenes and aromatics, but themechanisms involved in these cases are of so complicated a nature thatvery little positive information has been evolved in spite of the largeamount of experimentation which has been done and the large number oftheories proposed. In general, however, it mayv be said that, startingwith paraflln hydrocarbons representing the highest degree ofsaturation, these compounds are changed progressively into olefins,naphthenes, aromatics, and finally into carbon and hydrogen and otherlight fixed gases. It is not intended to infer from this statement thatany particular success has attended the conversion of any given paraiiinor other aliphatic using catalysts even in connection with converslonreactions among pure hydrocarbons and particularly in connection withthe conversion of the relatively heavy dlstillates and residua which areavailable for cracking, there is a general tendency for thedecomposition reactions to proceed at a very rapid rate, necessitatingthe use oi extremely short time factors and very accurate control oftemperature and pressure to avoid too extensive decomposition. There arefurther difliculties encountered in maintaining the efliciency ofcatalysts employed in pyrolysis since there is usually a rapiddeposition of carbonaceous materials on their surfaces and in theirpores.

The foregoing brief review of the art of hydrocarbon pyrolysis is givento furnish a general background for indicating the improvement in suchprocesses which is embodied in the present invention, which may beapplied to the treatment of pure paraflin or olefin hydrocarbons,hydrocarbon mixtures containing substantial percentages of paraffinhydrocarbons such as relatively close out fractions producible bydistilling petroleum, and analogous fractions which contain unsaturatedas well as saturated straight chain hydrocarbons, such fractionsresulting from cracking operations upon the heavier fractions ofpetroleum.

In one specific embodiment the present invention comprises theconversion of aliphatic hydrocarbons including paraifin and olefinhydrocarbons into aromatic hydrocarbons by subjecting them at elevatedtemperatures of the order of 400-700 C., to contact for definite timesof the order of 6-50 seconds with catalytic materials comprising majorproportions of aluminum oxide of relatively low catalytic activitysupporting minor proportions of oxides of elements selected from thoseoccurring in the lefthand columns of group VI of the periodic table,these oxides having relatively high catalytic activity.

According to the present invention aliphatic or straight chainhydrocarbons having 6 or more carbon atoms in chain arrangement in theirstructure are specifically dehydrogenated in such 6 a way that the chainof carbon atoms undergoes ring closure with the production in thesimplest case of benzene from *n-hexane or n hexene'and in the case ofhigher molecular weight parafllns of various alkyl derivatives ofbenzene. Under properly controlled conditions of times of contact,temperature and pressure, very high yields of the order of '75 to 90% ofthe benzene or aromatic compoimds are obtainable which are far in excessof any previously obtained in the art either with or without catalysts.For the sake of illustrating and exemplifying the types of hydrocarbonconversion reactions which are speciflcally accelerated under thepreferred conditions by the present types of catalysts, the followingstructural equations are introduced:

on, cm on on cfii cfi n-mana on. com Cfi: \CEICHI C6. \CH +m m cm on ona c s C 'tolnsns' on. on 06: om-cm on c-cm m om-cn. on -cm I n-octallso-xylanc In the foregoing table the structural formulas of the primaryparafiin hydrocarbons have been represented as a nearly closed ringinstead of by the usual linear arrangement for the sake of indicatingthe possible mechanisms involved. No attempt has been made to indicatethe possible intermediate existence of mono-oleflns, dioleiins,hexamethylenes or alhlated hexamethylenes which might result from theloss of various amounts of hydrogen. It is not known at the present timewhether ring closure occurs at the loss of one hydrogen molecule orwhether dehydrogenation of the chain carbons occurs so that the iirstring compound formed is an aromatic such as benzene or one of itsderivatives. The above three equations are of a relatively simplecharacter indicating generally the type of reactions involved butinthecaseof w mono-oleilns of higher molecular weight thantheoctaneshownandinthecaseofbranched chain compounds which containvarious alkyl substituent groups in different positions along thesix-carbon atom chain, more complicated reactions will be involved. Forexample, in the case oi such aprimary compound as 2,3-dimethyl hexanethe principal resultant product is apparenfly o-xylene although thereare concurrently produced deiinite yields of such compounds as ethylbenzene indicating an isomerizaflon of two suhstituent methyl groups. Inthe case of nmianes which are represented by the 2,3,4-trlmethyl hexane,there is formationnotonlyotmesitylenebutalsoofsuch compounds as methylethyl benzol and various propyl bensols.

Itwillbeseenfromtheforegoingthatthe scope of the present invention ispreferably limited to the treatment of aliphatic hydrocarbons whichcontain at least 6 carbon atoms in straight chain arrangement. In thecase of paraflin hydrocarbons containing less than 6 carbon atoms inlinear arrangement, some formation of aromatics may take place due toprimary isomerization reactions although obviously the extent of thiswill vary considerably with the type of compound and the conditions ofoperation. The process is readily applicable to paramns from hexane upto dodecane and their corresponding oleflns. With increase in molecularweight beyond this point the percentage of undesirable side reactionstends to increase and yields of the desired alkylated aromatics decreasein proportion.

According to the present invention composite catalytic materials areemployed which comprise in general maior proportions by weight ofgranular activated aluminum oxide as a base catalyst or supportingmaterial for minor proportions of oxides of the elements in the lefthandcolumn of Group VI of the periodic table comprising the elementschromium, molybdenum and tungsten. The base material comprising aluminumoxide is of relatively low catalytic activity while the oxides of theelements mentioned are of relatively high catalytic activity and furnishby far the greater proportion of the observed catalytic effects. Theoxides. of these several elements vary somewhat in catalytic activity inany given reaction comprised within the scope of the invention and thisvariation may further vary in the case of different types ofdehydrogenation and cyclization reactions. Some of the properties ofthese catalytically active oxides, which are developed on the surfaceand in the pores of the alumina particles will be described insucceeding P agraphs.

It should be emphasized that in the field of catalysis there have beenvery few rules evolved which would enable the prediction of whatmaterials would catalyze a given reaction. Most of the catalytic workhas been done on a purely empirical basis, even though at times certaingroups of elements or compounds have been found to be more or lessequivalent in accelerating certain types of reactions.

Aluminum onde which is generally preferable as a base material for themanufacture of catalysts for the process may be obtained from naturalaluminum oxide minerals or ores such as Bauxite or carbonates such asDawsonite by proper calcination, or it may be prepared by precipitationof aluminum hydroxide from solutions of aluminum sulfate or differentalums, and dehydration of the precipitate of aluminum hydroxide by heat.Usually it is desirable and advantageous to further treat it with air orother gases, or by other means to activate it prior to use.

Two hydrated oxides of aluminum occur in nature, to-wit, Bauxite havingthe formula AI:O:.2H:O and Diaspore AhOsIIsO. In both of these oxidesiron sesqui-oxide may partially replace the alumina. These two mineralsor corresponding oxides produced from precipitated aluminum hydroxideare particularly suitable for the manufacture of the present type ofcatalysts and in some instances have given the best results ofanyofthebasecompoundswhoseuse is at present contemplated. The mineralDawsonite having the formula NaaAMCOa) 3.2A1(OH).isanothermineralwhichmaybeusedasasource of aluminum oxide.

It is best practicein the final steps of preparing aluminum oxide as abase catalyst to ignite it for some time at temperatures within theapproximate range of from BOO-900 C. ably does not correspond tocomplete dehydration of the hydroxide but apparently gives a catalyticmaterial of good strength and porosity so that it is able to resist fora long period of time the deteriorating eii'ects of the service andregeneration periods to which it is subjected.

My investigations have also definitely demonstrated that the catalyticefficiency of alumina, which has some catalytic potency in itself isgreatly improved by the presence of oxides of the preferred elements inrelatively minor amounts, usually of the order of less than 10% byweight of the carrier. It is most common practice to utilize catalystscomprising 2 to 5% by weight of these oxides, particularly the loweroxides.

The oxides which constitute the principal active catalytic materials maybe deposited upon the surface and in the pores of the activated aluminagranules by several alternate methods such as for example, the ignitionof nitrates which have been adsorbed or deposited from aqueous solutionby evaporation or by a similar ignition o1 precipitated hydroxides. Asan alter-' native method though obviously less preferable, the flnelydivided oxides may be mixed mechanically with the alumina granuleseither in the wet or the dry'condition. The point of achieving the mostuniform practical distribution of the oxides on the alumina shouldconstantly be borne in mind since the observed catalytic effectsevidently depend principally upon a surface action.

The element chromium has three oxides, the trioxide CrOa, the dioxideClOz and the sesquioxide Cram, the last-named being readily produced byheating the trioxide in hydrogen or hydrocarbon vapors at a temperatureof 250 C. The dioxide has been considered to be an equimolecular mixtureof the trioxide and the sesquioxide. The oxides are readily developed onthe surfaces and pores of alumina granules by utilizing primarysolutions of chromic acid I-IzCrOr or chromium nitrate Cr(NO3):. Theignition of the chromic acid, the nitrate or a precipitated trihydroxideproduces primarily the trioxide which is then reduced to the sesquioxideto furnish an active catalyst for use in reactions of the presentcharacter.

The two most important oxides of molybdenum which are alternativelyemployed as catalysts according to the present invention are the dioxideM00: and sesquioxide M0201. Since thereduction 01' the trioxide byhydrogen begins at 300 C. (572 F.) and the reduction is rapid at 450 C.(842 F.) the effective catalytic material is principally thesesquioxide. The trioxide may be added to the active alumina carrierfrom a-solution in aqueous ammonia or from a solution of ammoniummolybdate which are added in amounts just requisite to wet the carriergranules uniformly and the mass is then dried and ignited.

The element tungsten has three oxides: the trioxide W03, the dioxide W0:and the sesquioxide W201. The trioxide is readily soluble inaqueousammonia from which it may be deposited upon active alumina granules andit is ordinarily reduced preliminary to service by the action ofhydrogen at a red heat. Tungstic acids may be precipitated from aqueoussolution to form the hydrated oxides and these may be heated to drive onwater and leave a residue of oxides on the carrier particles.

catalytic activity, its scarcity precludes its extensive use inpractice.

It has been found essential to the production of high yields ofaromatics from aliphatic hydrocarbons when using the preferred types ofcatalysts that depending upon the aliphatic hydrocarbon or mixture ofhydrocarbons being treated, temperatures from 400-700 C. Should beemployed, contact times of approximately 6 to 50 seconds and pressuresapproximating atmospheric. The use of subatmospheric pressures of theorder of atmosphere may be beneficial in that reduced pressuresgenerally favor selective dehydrogenation reactions but on the otherhand moderately superatmospheric pressures usually of the order of lessthan 100 lbs., per square inch tend to increase the capacity ofcommercial plant equipment so that in practice a balance is struckbetween these two factors. The times of contact most commonly employedwith n-paraflinic or mono-oleflnic hydrocarbons having from 6-12 carbonatoms to the molecule are of the order of 8-20 seconds. It will beappreciated by those familiar with the art of hydrocarbon conversion inthe presence ofcatalysts that the factors of temperature, pressure andtime will frequently have to be adjusted from the results of preliminaryexperiments to produce the best results in any given instance. Thecriterion of the yield of aromatics will serve to fix the bestconditions of operation. In a general sense the relations between time,temperature and pressure are preferably adjusted so that ratherintensive conditions are employed oi suflicient severity to insure amaximum amount of the desired cyclization reactions with a minimum ofundesirable side reactions; It too short times of contact are employedthe conversion reactions will not proceed beyond those of simpledehydrogenation and the yields of oleilns and dioleilns will predominateover those of aromatics.

While the present and cost naturally may also be employed to producearomatics from aliphatic hydrocarbon mixtures such as distillates fromparafllnic or mixed base crude petroleum. In this case the aromaticcharacter of the distillates will be increased and as a rule the octanenumber will be higher. If desired and found feasible on a basis ofconcentration, the aromatics produced in the hydrocarbon mixtures may berecovered as such by distillation into fractions of proper boiling rangefollowed by chemical treatment with reagents capable of reactingselectively with them. Another method of aromatic concentration willinvolve the use of selec-- tive solvents such as liquid hols, furfural,chlorex, etc.

In operating the process the general procedure is to vaporizehydrocarbons or mixtures 01' hydrocarbons and after heating the vaporsto a suitable temperature within the ranges previously specified, topass them through stationary masses of granular catalytic material invertical cylindrical treating columns or banks of catalyst-containingtubes in parallel connection. Since the reactions are endothermic it maybe necessary to apply some heat externally to mainsuli'ur dioxide,alcolytic oxides due to preferential tain the best reaction temperature.After passing through the catalytic zone the products are submitted tofractionation to recover cuts or fractions containing the desiredaromatic product with the separation of fixed gases, unconvertedhydrocarbons and heavier residual materials, which may be disposed of inany suitable manner depending upon their composition. The overall yieldof aromatics may be increased by recycling the unconverted straightchain hydro carbons to further treatment with fresh material, althoughthis is a more or less obvious expedient and not specificallycharacteristic of the present invention.

It is an important feature of the present .procass that the vaporsundergoing dehydrogenation should be free from all but traces of watervapor since the presence of any substantial amounts of steam reduces thecatalytic selectivity of the composite catalyst to a marked degree. Inview of the empirical state of the catalytic art, it is not intended tosubmit a complete explanation of the for the deleterious influence ofwater vapor on the course of the present type of catalyzed reactions,but it may be suggested that theactionofthesteammaybeto causeapartialhydration of alumina and some of the cataadsorption so that in effectthe hydrocarbons are prevented from reaching or being adsorbed by thecatalytically active surface.

The present types of catalysts are particularly effective in removinghydrogen from chain compounds in such a way that cyclization may bepromoted without removal of hydrogen from end carbon atoms so that bothend and side alkyl groups may appear as substituents in benzene ringsand it has been found that under proper operaidng conditions they do nottend to promote any great amount of undesirable side reactions leadingto the deposition of carbon or carbonaceous materials and for thisreason show reactivity over relatively long periods of time.

When their activitybegins to diminish met a period of service, it isreadily regenerated by the simple expedient of oxidizing with air orother oxidizing gas at a moderately elevated temperature, usually withinthe range employed in the dehydrogenation and cyclization reactions.This oxidation effectively removes traces of carbon deposits whichcontaminate the surface of the particles and decrease their efliciency.It is characteristic of the present types of catalysts that they may berepeatedly regenerated with only a very gradual loss of catalyticemciency.

During oxidation with air or other oxidizing gas mixture in regeneratingpartly spent material, there is evidence to indicate that the loweroxides are to a large extent, if not completely, oxidized to higheroxides which combine with aluminum oxide to form aluminum salts of variable composition. Iater these salts are decomposed by contact withreducing gases in the first stages of service to reform the lower oxidesand regenerate the real catalyst and hence the catalytic activity.

Example 1 In this example an ultimate yield of over 90% benaol wasproduced by the catalytic conversion of a n-haxane fraction obtainedfrom a highly c crude petroleum by close fractionation. The catalystcomprised an alumina base supporting about 4% by weight of chromiumsesquioxide which had been developed on the carrier particles by theignition of chromium nitrate and mass the reduction of the primarytrioxide by hydrogen at a temperature of 250-300 C. The hexane fractionwas passed through a bed of this catalyst at a temperature of 525 0.,atmospheric pressure and a time of contact of 20 seconds to produce aonce-through yield of about 47%. The final yield after recyclingunconverted hexane several times was above 90% as previously stated.

Example 11 In this case n-heptane was converted to toulene utilizing acatalyst supporting molybdenum oxides on the preferred alumina base. Thecatalyst was made by utilizing a solution of ammonium molyhdate in anexcess of ammonia and adding the concentrated solution to about threetimes its weight of granular alumina particles followed by carefulmixing and calcining to drive of! water and ammonia and leave a residueof the trioxide. Before service the particles were treated with hydrogenat about 450 C. to reduce a material portion of the trloxide to loweroxides such as the sesquioxide.

n-Heptane was passed over a bed of the catalyst particles at atemperature of 555 C., atmospheric pressure and 13 seconds contact timeto produce a yield of approximately 50% of toluene on a once-throughbasis, this yield being finally raised to about 80% by completerecycling of unconverted material.

Example III This example is given to illustrate the direct formation oftoluene from n-heptane, which conversion was accomplished using thecatalyst similar to that described under Example 11, a temperature of510 C., atmospheric pressure and a time of contact of approximately 20seconds. The once-through yield of toluene was 76% and the ultimateyield was in the neighborhood of 93-95% by recycling unconvertedolefin.-

Example IV To prepare the catalyst an ammoniacal aqueous solution oftungsten trloxide was used to deposit the trioxide upon an activatedalumina.

After reduction with hydrogen, analyses showed, there was present from45% of mixed tungsten oxides.

Using the above catalyst the vapors of n-heptane were treated at atemperature of 560 C., substantially atmospheric pressure, and 15seconds contact time to produce a yield of 46% to toluene on aonce-through basis which was finally brought to about a 76% ultimateyield after several recyclings of unconverted charge.

The foregoing specification and examples show clearly the character ofthe invention and the results to be expected in its application toallphatic hydrocarbons, although neither section is intended to beunduly limiting.

I claim as my invention:

1. A process for the production of aromatic hydrocarbons from aliphatichydrocarbons of from six to twelve carbon atoms, which comprisesdehydrogenating and cyclicizing the aliphatic hydrocarbon by subjectionto a temperature of the order of 400 to 700 C., for a period of about 6to 50 seconds, in the presence of an aluminum oxide catalyst containinga relatively small amount of an oxide of a metal from the left handcolumn of Group VI of the periodic table and selected from the classconsisting of chromium, molybdenum, tungsten and uranium.

2. A process for the production of aromatic 1g hydrocarbons from;aliphatic hydrocarbons oi from six to twelve carbon atoms, whichcomprises dehydrogenating and cycliclzing the aliphatic hydrocarbon bysubjection to a temperature of the order of 400 to 700 C., for a periodabout 6 to 50 seconds, in the presence of an aluminum oxide catalystcontaining a relatively small amount of an oxide of chromium.

3. A process for the production of aromatic hydrocarbons irom aliphatichydrocarbons of from six to twelve carbon atoms, which co'nprisesdehydrogenating and cyclicizing the allphatic hydrocarbon by subjectionto a temperature of the order of 400 to 700 C., for a period of about 6to 50 seconds, in the presence of an aluminum oxide catalyst containinga relatively small amount oi. an oxide of molybdenum.

4. A process for the production of aromatic hydrocarbons from aliphatichydrocarbons of from six to twelve carbon atoms, which comprisesdehydrogenating and cyclicizing the aliphatic hydrocarbon by subjectionto a temperature of the order oi 400 to 700 C., for a period of about 6to 50 seconds, in the presence of an aluminum oxide catalyst containinga relatively amount of an oxide of tungsten.

5. A process for the production of aromatic hydrocarbons from aliphatichydrocarbons of from six to twelve carbon atoms, which comprisesdehydrogenating and cycliclzing the allphatic hydrocarbon by subjectionto a temperature of the order of 400 to 700 C., for a time period oflessfthan 50 seconds but suillcient to dehydrogenate and cyclicize thealiphatic hydrocarbon, in the presence oi analuminum oxide catalystcontaining a relatively small amount of an oxide of a metal from theleft hand column of Group VI of the periodic table and selected from theclass consisting of chromium, molybdenum, tungsten and uranium.

8. A process tor the production of aromatic hydrocarbons from aliphatichydrocarbons 01 from six to twelve carbon atoms, which comprisesdehydrogenating and cyclicizing the allphatic hydrocarbon by subjectionto a temperature of the order of 400 to 700 C., for a time period ofless than 50 seconds but suiilcient to dehydrogenate and cyclicize thealiphatic hydrocarbon, in the presence of an aluminum oxide catalystcontaining a relatively small amount of an oxide of chromium.

7. A process for the production of aromatic hydrocarbons from aliphatichydrocarbons of from six to twelve carbon atoms, which comprisesdehydrogenating and cyclicizing the aliphatic hydrocarbon by subjectionto a temperature of the order of 400 to 700 C., for a time period ofless than 50 seconds but suflicient to dehydrogenate and cyclicize thealiphatic hydrocarbon, in the presence of an aluminum oxide catalystcontaining a relatively small amount or an oxide of molybdenum.

8. A process for the production of aromatic hydrocarbons from aliphatichydrocarbons oi! from six to twelve carbon atoms, which comprisesdehydrogenating and cycllcizing the allphatic hydrocarbon by subjectingto a temperature oi the order of 400 to 700 C., for a time period orless than 50 seconds but sufllcient to dehydrogenate and cyclicize thealiphatic hydrocarbon, in the presence 0! an aluminum axide catalystcontaining a relatively small amount of an oxide of tungsten.

ARISTID V. GROSSE.

